A clutch cover assembly is generally mounted to the flywheel of an engine, and is used to transmit the drive force of the engine to the transmission side. This clutch cover assembly mainly includes a clutch cover that is fixed to the flywheel, a pressure plate that sandwiches the friction member of the clutch cover assembly between itself and the flywheel, and a diaphragm spring that presses the pressure plate to the flywheel side. The diaphragm spring is made of an annular elastic component and a plurality of levers extending from the annular elastic component to the inside in the radial direction. The diaphragm spring functions to press on the pressure plate, and also functions to release the pressing against the pressure plate.
The pressing load characteristics of a clutch cover assembly will be described. The pressing load curve expresses a usage region as a pressing load in the load curve of a diaphragm spring. For example, as shown in FIG. 5, which is a graph of the pressing load curve 20, the effective usage region of the clutch cover assembly (wear allowance) is the region over which a specific pressing load can be obtained (from the set line 25 of a new part to the wear line 26 at which the friction member reaches the wear limit).
Next, the release load curve of the clutch cover assembly will be described. The release load curve expresses the relationship between the amount by which a release lever is operated (the lever stroke) and the load acting on the distal end of the release lever (the release load). For example, as shown in FIG. 6, a release load curve 60 has a first portion 61 that increases linearly from a lever operation amount of zero, and a second portion 62 that smoothly decreases. The first portion 61 expresses the lever stiffness of the diaphragm spring, while the second portion 62 corresponds to a change in the release load curve from the set line to the right in the drawing.
As shown in FIG. 5, the pressing load curve 20 rises at a constant rate as the amount of displacement of the diaphragm spring increases from zero, but once the amount of deflection passes a certain point (the peak point), the curve falls smoothly thereafter, and when another amount of deflection is exceeded, rises smoothly. Therefore, within the effective usage region, the curve is a peaked portion 21 (the portion that is convex at the top), and the pressing load increases as the wear of the friction member increases (as the set line moves to the left in the drawing). That is, as the friction member wears down, the release load increases, and the driver has to exert a greater pressing force on the clutch pedal.
In view of this, one structure known in the past for cutting the peak in the pressing load curve was a peak-cut clutch, in which an elastic member is used to generate a load to counteract the load of the diaphragm spring when the friction member wears down. With a peak-cut clutch, the opposite-facing peaked portion of the curve of the elastic member is superimposed onto the peaked portion of the curve of the diaphragm spring, and as a result a flat portion is obtained in the composite load (see Japanese Laid-Open Utility Model Application H3-22131, for example).